Electrical resistance device



March 21, 1967 a. e. DOWNING ETAL 3,310,766 I ELECTRICAL RESISTANCEDEVICE Filed July 14, 1965 FIG. 1

l/V VE/V TOES L50 6; DOWN/N6, JAMES M K/A/DA/J.

United States Patent 3,310,766 ELECTRICAL RESISTANCE DEVICE Leo G.Downing and James V. Kindall, Riverside, Calih,

assignors to Bourns, Inc. Filed July 14, 1965, Ser. No. 472,002

7 Claims. (Cl. 338-) The invention herein described pertains toelectrical resistance devices and more particularly to improvements inthat class of resistance devices constructed and arranged to provide anoutput thatis exponentially related to the input potential or appliedvoltage.

There are on the market voltage-variable electrical resistance devices,known as varistors, which are characterized by a negative temperaturecoefficientot resistance. It is known in the art to assemble a pluralityof varistors in series relationship, as by stacking a plurality of thedisc-like devices in a cylindrical container with a corrective orcompensating network in series, to provide an impedance unitcharacterized by an output potential that is functionally related to aninput potential by other than a linear relationship, such as, forexample an exponential relationship. Thus by judicious selection ofvaristors, a desired functional relationship, such as the cosine, or thesine, or mathematical square, may be attained. Means and modes forattaining such results are, for example, set out in US Letters PatentNo. 3,211,901, issued Oct. 12, 1965, the disclosure of which isincorporated herein by reference, to any extent necessary to a completeunderstanding of the present invention.

In the prior are exponential resistors or resistance devices hereinabovereferred to, the polycrystalline semiconductive materials, formed asdisc-like tablets of selected material such as carbide crystalscompacted into a unitary mass and bearing a solder-coated flashing ofcopper on the 'faces, were disposed in a cylindrical housing under somecompression so as to maintain them in series relationship. As soconstructed and arranged, very slight movements of the discs relative toeach other and relative to the terminal discs occurred incident tochanges of temperature and incident to the resistor being vibrated inuse. Such movements naturally changed the disc-todisc contactresistance, and thus was the cause of electrical noise in the circuitcontaining the resistor and also was the cause of gross variations ofthe terminalto-termi-nal resistance,

The present invention overcomes the aforementioned objectionablecharacteristics or features of the prior art nonlinear impedance units,and provides a unit comprising a series or group of polycrystallinesemiconductive devices such as varistors electrically in seriesarrangement, in which no appreciable change in the disc-to-discresistance occurs irrespective of repetitive changes in temperature overa wide range and irrespective of severe vibration. According to theinvention, the disc-like semiconductive units (such as varistors) areselected as may be required to attain the desired variationof resistancewith variation of applied potential, and are assembled into a stack, inaxial alignment, with asmall drop of highly conductive, highly adhesiveself-curing conductive cement between each unit and that next adjacentthereto. Thereafter, the disc-like units are maintained inpressedtogether relationship in axial alignment, as'by being held in acylinder, during curing (hardening) of the applied cement. Thus aunitary mass, comprising a series of series-connected nonlinearresistors, is produced. The resulting integral unit is characterized bysubstantially zero electrical resistance between contacting faces of anypair of the stacked devices due to the highly conductive cement; and isfurther characterized by extreme constancy of the inter-resistor orface-to-face contact re- 3,310,766 Patented Mar. 21, 1967,

sistance during heat or temperature cycling and during application ofvibratory stresses. As will be evident, the areal extent of interfacecontact between two of the disc-like resistive devices is very largerelative to the over all size and/or mass of one of the devices, wherebythe forces of the adhesive bond are very large as compared with anyexternally applied opposing force, and the bonds are thus highlyeffective in maintaining the devices in fixed electrical and mechanicalrelationship in the integral group. As may be expedient and desirable,the integrated stack of semiconductive resistive devices may be housedin a closely fitting insulated tube in which may also be enclosed thefixed-value resistors, compensating means, terminal devices andinsulators, all as hereinafter described in greater detail.

The preceding brief general description of the invention makes itevident that a principal object of the invention is to provideimprovements in nonlinear voltage-variable resistor units.

, Another object is to provide an integrated resistor unit comprising aplurality of voltage variable resistors, which unit is characterized byelectrical immunity to extreme vibration and heat-cycling.

Another object of the invention is to provide an int egral unitcomprising a stack of face-to-face varistors, the unit beingcharacterized by freedom from face-toface electrical contact noise.

Another object of the invention is to provide a stack of disc-likepolycrystalline semiconductive resistors which are both mechanicallybonded together to form an integral unit and electrically interconnectedinseries by a solid electrically conductive medium.

Other objects and advantages of the invention are hereinafter stated ormade evident in the appended claims and following description of apreferred form of structure accordingto the invention, reference beingmade in the description to the appended drawings forming a part of thisspecification.

In the drawings:

FIGURE 1 is a. pictorial view, to no particular scale, of a.voltage-variable nonlinear resistive device incorporating an integratedunit of electrical resistors, according to the invention;

FIGURE 2 is a longitudinal cross-sectional view of the device depictedin FIGURE 1 but to larger scale, with incidental structures notsectioned;

'FIGURE 3 is a pictorial view, to an arbitrary scale, showing a singlevaristor device of the general character f those used in a structureaccording to the invention, with a part cut away;

FIGURE 4 is a sectional view of structure comprised in the resistivedevice depicted in FIGURE 1, the section being taken as indicated bybroken line and arrows at 44 in FIGURE 2; and

.FIGURE 5 is a view similar to FIGURE 4 but taken as indicated at brokenline 55 in FIGURE 2.

Referring now to the drawings and to FIGURE 1 in particular, theresistive device is denoted generally by ordinal 10. It comprises acylindrical shell 12, end closures 14 and 16 (FIGURE 2) and terminalwires or pigtails 18 and 20, with other components enclosed in thehousing formed by the shell and end closures.

As depicted in FIGURE 2, the device includes among other componentsdisposed within shell 12, a stack of;

exemplary disc-like polycrystalline semiconductive resistive deviceswhich in this case are chosen to be silicon carbide varistors denoted by'ordinal22. As indicated in FIGURE 3, each varistor comprises a roundfiat-faced 7 disc of shape-retaining resistive silicon carbide crystalscompacted with a binder of, for example, thermosetting synthetic resin.The varistor comprises at each face thereof a very thin layer or flashof copper 22c which may cover all of the face or as shown only a ma orpart of such face; and overlying at least some of the copper at eachface a respective coating of solder or tin 22s. Thus the individualvaristor has a body and two terrnlnals, the latter being constituted bythe applied copper and tin. The terminals of next-adjacent varistors ina stack are thus arranged and of a nature favorable for effecting a goodseries electrical interconnection of the contacting varistors. In theillustrations the thicknesses of the applied flashing and tin or solderare grossly exaggerated, for convenience of illustration. In practicethe applied materials are very thin.

According to the invention and as best illustrated 1n FIGURE 2, a stackof the polycrystalline semiconductive resistors 22 is assembled incoaxial arrangement, with a drop of conductive hardenable adhesive 24between each pair of abutting resistor terminals. As the resistors arepressed together in the stack the adhesive spreads and in each casecovers substantially all of the tin or solder 22s and preferably also atleast some of the copper 22a, to form a film-like layer bonding theresistors together. The adhesive film is depicted in FIGURE 2 grosslyenlarged in thickness for convenience of illustration, it beingunderstood that the film is such as to permit the abutting solder layersto come into contact at a plurality of points or areas but elsewherefilling the interface voids and adhesively binding the resistor discstogether into a unitary mass. The adhesive, formed preferably of silverflake and thermosetting resin, is permitted to cure at room temperaturewhile the assembled stack of resistors is maintained in a cylindricalarray under moderate compressive forces. One satisfactory conductiveadhesive may be prepared by stirring 80 grams of silver flake (MetalsDisintegrating Corporation MD 750, for example) into grams of ShellChemical Corporation Epon 815 epoxy resin, and using as a catalyst forthe resin 2 grams triethylene tetramine (e.g., 2 grams TETA marketed byUnion Carbide Corporation).

Following curing or hardening of the conductive adhesive 24 the unitarymass comprising the adhesively-bonded varistors 22 has soldered toend-most faces thereof respective terminal devices including wires orpigtails 18 and 18a (FIGURE 2), the terminal devices preferablyincluding termination shims 26, 26 formed of metal which areconductively united with respective films 22s at the stack ends and towhich the wires are soldered as indicated. The thus-equipped unitaryassembly of varistors is closely jacketed by an insulation cover 28(FIGURE 2) whereby the unit may be inserted into the cylindrical shell12 and potted therein with potting compound 14p. An insulative disc .14dserves with nonconductive potting compound 14p to provide the endclosure 14 which effectively seals the end of the shell against ingressof foreign material and concurrently maintains the active components ofthe device in place. As is made evident in FIGURE 2, the terminal wire18 extends through an aperture in disc 14d.

At its opposite end the unitary stack of varistors is connected via theconductor (wire or pigtail 18a) to a low-voltage compensation deviceincluding in this exemplary device a pair of etched-circuit terminalboards 30, 32 of disc-like form and compensating network means 34therebetween which network comprises diode devices. The compensatingnetwork may be of divers sorts and per se is not of the presentinvention. Following connection of pigtail 18a to the compensatingnetwork, nonconductive potting compound 14p poured into the housing 12over the unitary stack of varistors 22 and the compensating network withattached terminal boards is pressed into thehousing. Potting of theentire structure in the housing is then completed in any suitablefashion, as by addition of nonconductive compound and preferably usingvacuum for removal of entrapped gas. The housing is completed byemplacement of end cap 14d as indicated in FIGURE 2.

As will now be evident, the unitary structure comprising the stack ofadhesively-bonded resistors is securely fixed in place in housing shell12, and is such that change of inter-resistor connection resistance dueto mechanical movement cannot occur. The structure is sealed againstingress of foreign material, and is protected from mechanicaldisplacements or injuries. Thus the aforementioned objects of theinvention have been attained. In the light of the disclosure, changeswithin the true spirit and scope of the invention will be suggested toothers, and hence it is desired that the invention be restricted only asrequired by the appended claims;

We claim:

1. A nonlinear electrical resistance device comprising:

a series of electrical resistors each comprising a disclike structurecomposed principally of silicon carbide crystals and a binder, each ofthe series of resistors bearing a conductive metal coating on at least amajor portion of a face thereof, said resistors being stacked in faceto-face relationship in a generally cylindrical array;

a series of thin masses of highly conductive hardened adhesive, each ofsaid masses being disposed between the next-adjacent faces of arespective pair of said resistors and serving to electricallyinterconnect the respective pair of resistors and the masses of adhesiveserving to firmly mechanically bond the series of resistor-s into arigid unitary structure; and

end terminal device-s arranged at respective ends of said unitarystructure to provide electrical connection thereto.

2. A resistance device according to claim 1, in which said end terminaldevices each comprise a conductive disc adhesively secured to arespective end of said series of resistors, and conductor means eachfusion-united to a respective conductive disc.

3. An electrical resistance device comprising:

first means, including a series of resistive disc like devices eachcomprising essentia-lly a compacted mass of polycrystallinesemiconductive material and a binder, said disc-like devices beingarranged in a faceto-face stack arrangement;

second means, including between opposed faces of said disc-like devicesrespective masses of conductive adhesive serving concurrently to effectstable electrical interconnections between the said disc-like devicesand to effect stable mechanical bonds whereby the said disc-like devicesare bonded into a rigid unitary electrical device; and

third means, comprising electrical terminal means electrically connectedto respective end-most faces of said stack of said disc-like devices,

whereby the said disc-like devices are electrically connected in seriesrelationship and whereby the interdisc electrical resistances of saidstack of said disclike devices remain stable incident to said devicebeing subjected to severe vibration.

4. A resistance device according to claim 3, in which saidpolycrystalline sem'iconductive material is at least principally siliconcarbide.

5. An electrical resistive device comprising:

first :means, including a stack of resistive semiconductivepolycrystalline elements of disc-like form the internal face-to-faceelectrical resistance of which re-- sistive elements varies nonlinearlywith variation of applied potential, said resistive elements beingarranged generally in faceto-face contact;

second means, including a series of masses of conductivemetal-containing polymer adhesive each of said masses being disposedbetween and in electrical contact with the contacting faces of arespective pair of said resistive elements and serving to improve theconductivity of the electrical path therebetween and further serving toadhesively bind the respective pair of resistive elements together,whereby the said stackof resistive elements is firmly united into arigid unitary structure with the resistive elements in electrical seriesrelationship; 7

and third means, including means housing said resistive elements andproviding electrical terminal connections thereto.

6. A resistive device according to claim 5, including fourth means, saidfourth means including electrical compensating circuit means disposed insaid housing and connected to at least one of said resistive elements.

7. An exponential resistive device characterized by an exponentiallyvarying current flow in response to linearly varying applied potential,said device comprising:

first means, including a plurality of disc-shaped resistive members eachcomposed essentially of a compacted mass of selected grains ofcrystalline silicon carbide and means binding the grains into a coherentdisc-shaped mass, said members being arranged in a stack with nextadjacent members in face toface contact;

second means, including a plurality of masses of highly conductiveadhesive each disposed between the contacting faces of a respective pairof said members 'and serving to adhesively bind the said members into arigid unitary mas-s and to improve and stabilize the fa ce-todaceconductivity between the respective pair of members; third means,including generally cylindrical housing means including insulationmeans, serving to en- 5 close and insulate said first and second means;and

fourth means, including electrical terminal means extending fromrespective ends of said housing means, effective to provide electricalconnections to the endmost ones of said stack of resistive members.

References Cited by the Examiner UNITED STATES PATENTS FOREIGN PATENTSCanada.

RICHARD M. WOOD, Primary Examiner.

25 W. D. BROOKS, Assistant Examiner.

5. AN ELECTRICAL RESISTIVE DEVICE COMPRISING: FIRST MEANS, INCLUDING A STACK OF RESISTIVE SEMICONDUCTIVE POLYCRYSTALLINE ELEMENTS OF DISC-LIKE FORM THE INTERNAL FACE-TO-FACE ELECTRICAL RESISTANCE OF WHICH RESISTIVE ELEMENTS VARIES NONLINEARLY WITH VARIATION OF APPLIED POTENTIAL, SAID RESISTIVE ELEMENTS BEING ARRANGED GENERALLY IN FACE-TO-FACE CONTACT; SECOND MEANS, INCLUDING A SERIES OF MASSES OF CONDUCTIVE METAL-CONTAINING POLYMER ADHESIVE EACH OF SAID MASSES BEING DISPOSED BETWEEN AND IN ELECTRICAL CONTACT WITH THE CONTACTING FACES OF A RESPECTIVE PAIR OF SAID RESISTIVE ELEMENTS AND SERVING TO IMPROVE THE CONDUCTIVITY OF THE ELECTRICAL PATH THEREBETWEEN AND 